An automobile bumper is usually made of an exterior surface skin, a cushioning body to absorb shocks, and a reinforcement adapted to reduce elastic deformation. The bumper is made of metal or plastics. The metal bumper is heavy and is permanently deformed when hit. The plastics bumper is free of such a disadvantage.
While an injection-molded plastics bumper has a smooth surface, it has a lower impact-resistance and thus needs a heavy, stiff reinforcement, making it difficult to reduce the weight of the entire bumper.
In contrast, a plastics bumper produced by blow molding is excellent in impact resistance (absorption of impact force): however, it has a rather rough surface, because it is made of a low-flow synthetic resin to provide drawdown resistance during the blow molding, and also because its blow pressure applied on the mold wall is so low that the contours of the mold are not fully transferred to the resin.